Many decades ago I used to go shark hunting with friends. The only handgun I had at that time was a .38spcl. revolver. The sharks just loved that revolver, as it had little effect on them. I bought a surplus carcano rifle and several hundred rounds of ammunition at Sears or Wards for less than twenty dollars. After that I had no problem getting one shot kills. I even shot a few in the water to good effect (at the surface). What we were doing was catching the sharks on a very heavy cable, winching them in, shooting them as they were up in the air, removing the hook and tossing them back in to attract more. Maybe your pistol would be effective on halibut - I never shot anything other than sharks.